Caption: Europa's ocean. illustration based on astronomical research studying the frozen surface of Europa, one of the four largest moons of Jupiter. A cutaway view of the ice crust covering Europa's liquid ocean fills the lower frame. Jupiter is at upper right. Io, another of Jupiter's moons, is at upper centre. A process similar to volcanism on Earth is producing eruptions on Europa's surface. Evidence of these eruptions is the presence of magnesium sulphates on the surface, thought to originate from chloride salts from Europa's oceans and sulphur that reaches Europa from volcanic eruptions on Io.